The device, which uses a technique called “incendiary reflection,” uses a camera to track facial expressions and subtly lifts the corners of the mouth and crinkles the area around the eyes.

Although smiles can of course be faked, seeing a cheerful expression can trigger a wave of happiness, according to Tara Kraft, a doctoral student at the University of Kansas who has studied facial feedback theory.

The mirror works by tapping into our impulse to mimic others’ facial expressions. Seeing a smile, which is registered in our minds as happy, makes us happy.

Japanese researchers, led by Shigeo Toshida, conducted an experiment on 21 volunteer subjects, asking them to perform a neutral task and then to rate how they felt while gazing, unbeknown to them, into the ‘liar mirror’.

Those who saw happier images of them said they felt a sense of euphoria, while some who saw a gloomier version reportedly felt somber.

The study also attached happiness to an impulse to buy things.

Another experiment showed that people who saw happier faces in the mirror while trying clothes on can “manipulate consumers’ impressions of products” and persuade them to purchase the item.